Classical cadherins constitute a family of single-pass transmembrane glycoproteins that mediate calcium-dependent cell adhesion. Members of this family include E-cadherin, N-cadherin, R-cadherin, and P-cadherin. Cadherins promote cell adhesion in a homophilic manner (a cadherin on the surface of one cell binds to an identical cadherin on the surface of another cell) (Takeichi, (1990) Ann. Rev. Biochem., 59, 237-252). Cadherins are not merely “biological glues”. They are also capable of regulating different signaling pathways by virtue of their ability to interact with a variety of receptor protein kinases (such as the fibroblast growth factor receptor), as well as their ability to bind the intracellular signalling protein, known as β-catenin (Qian et al., (2004) EMBO J., 23, 1739-1748).
The extracellular portion of classical cadherins is composed of five domains (EC1-EC5) of approximately 110 amino acids each separated by calcium binding sites. Calcium is absolutely required for the correct folding of the cadherin and confers a rod-like structure essential for adhesion with another cadherin molecule (Nagar et al., (1996) Nature, 380, 360-364). Cadherins are also composed of a single hydrophobic transmembrane domain and two cytoplasmic domains that link the cadherin molecule to different intracellular proteins such as β-catenin, γ-catenin and p120-catenin. These proteins in turn link cadherins to the cytoskeleton and other signaling molecules (Blaschuk et al., (2002) Mol. Membr. Biol., 19, 75-80).
Inappropriate cadherin expression by specific cells is one of the mechanisms for cancer development, cell invasion and metastasis. E-cadherin is primarily expressed in epithelial cells and acts as a tumor suppressor. In carcinomas, E-cadherin expression is reduced or lost by genetic or epigenetic events (Van Aken et al., (2001) Virchows Arch., 439, 725-751) and is accompanied by a loss of cell polarity and by cell dedifferentiation. Furthermore, transfection of carcinoma cells with E-cadherin cDNA results in a more differentiated and less invasive phenotype (Vleminckx et al., (1991) Cell, 66, 107-119). N-cadherin is expressed by many normal cell types including neural, vascular smooth muscle and endothelial cells, but not normal epithelial cells. N-cadherin is inappropriately expressed in many types of cancer. It has been reported that in carcinomas, loss of E-cadherin is accompanied by a de novo N-cadherin expression (Van Aken et al., (2001) Virchows Arch., 439, 725-751). In cancer cells, N-cadherin plays an important role in tumor cell invasion and metastasis (Hazan et al., (2000) J. Cell. Biol., 148, 779-790). In some cancer cells, E- and N-cadherin are coexpressed, but the contribution of N-cadherin to invasive potential takes precedence over E-cadherin function (Nieman et al., (1999) J. Cell. Biol., 147, 631-644). In addition, tumor angiogenesis is essential for tumor development and metastasis (Folkman et al., (1992) J. Biol. Chem., 267, 10931-10934) and targeting tumor vasculature represents a new approach to cancer treatment (Kelland, (2005) Cur. Cancer Therapy Rev., 1, 1-9). N-cadherin is expressed by pericytes (smooth muscle-like cells) and endothelial cells, thereby ensuring the interactions of these cells, and therefore plays an essential role in blood vessel formation and the maintenance of blood vessel stability (Gerhardt et al., (2003) Cell Tissue Res., 314, 15-23; Paik et al., (2004) Genes Dev., 18, 2392-2403). N-cadherin antagonists have potential for selectively disrupting the tumor vasculature without effecting normal blood vessels. Such antagonists therefore represent promising anti-cancer drugs.
Permeability barriers arising from cell adhesion create difficulties for the delivery of drugs to specific tissues and tumors within the body. For example, skin patches are a convenient tool for administering drugs through the skin. However, the use of skin patches has been limited to small, hydrophobic molecules because of the epithelial and endothelial cell barriers. Similarly, endothelial cells render the blood capillaries largely impermeable to drugs, and the blood/brain barrier has hampered the targeting of drugs to the central nervous system. In addition, many solid tumors develop internal barriers that limit the delivery of anti-tumor drugs and antibodies to inner cells. Permeability barriers also make it difficult to deliver viruses (such as adenoviruses) to tissues (such as lungs), thus hampering gene therapy approaches to the treatment of diseases.
Attempts to facilitate the passage of drugs across such barriers generally rely on specific receptors or carrier proteins that transport molecules across barriers in vivo. However, such methods are often inefficient, due to low endogenous transport rates and/or to the poor functioning of a carrier protein when coupled with drugs. While improved efficiency has been achieved using a variety of chemical agents that disrupt cell adhesion, such agents are typically associated with undesirable side-effects, may require invasive procedures for administration and may result in irreversible effects.
Thus, there remains a need for agents capable of modulating one or more of the various processes mediated by classical cadherins, such as N-cadherin and E-cadherin. The present invention identifies, using phage display technology, new classes of peptide compositions effective for modulating classical cadherin-mediated processes, and offers other related advantages. The present invention also provides methods for identifying and using new compounds capable of modulating cadherin-mediated processes.